Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates generally to downhole tools that control thrust generating members. More particularly, the present invention relates to an apparatus that absorbs the thrust generated by a downhole tool having a mud motor and/or a propulsion system.
2. Description of the Related Art
It is known that the recovery of subterranean deposits of hydrocarbons requires the construction of wells having boreholes hundreds, perhaps thousands, of feet in depth. One known system configured for well construction activities includes a bottom hole assembly (BHA) that is tethered to surface support equipment by a flexible umbilical. This BHA may be a self-propelled system that forms a borehole using a bit adapted to disintegrate the earth and rock of a subterranean formation. One such system is described in U.S. Pat. No. 6,296,066, entitled xe2x80x9cWell System,xe2x80x9d issued Oct. 2, 2001, hereby incorporated herein by reference for all purposes. This system preferably includes a bit, a downhole means to rotate the bit, and a downhole means to thrust the bit against the bottom of the borehole. An exemplary arrangement utilizes a positive displacement motor (e.g., a xe2x80x9cmud motorxe2x80x9d) to rotate the bit and a tractor to generate thrust or weight on bit (WOB). In these systems, high pressure drilling mud is conveyed to the BHA through the umbilical. After passing through the BHA, the drilling mud exits through nozzles located in the bit and the drilling mud with returns flows back to the surface via an annulus formed between the umbilical and the borehole wall. The mud motor and tractor use the drilling fluid flowing through the umbilical as their power source.
A system wherein two or more components share a common hydraulic fluid supply have certain drawbacks. Referring now to FIG. 1, there is schematically shown an exemplary hydraulic circuit that is susceptible to these drawbacks. The hydraulic circuit includes a fluid line 10, a tractor 11 having a pressure chamber 12 and piston head 13, a mud motor 14 having a power section 18 that includes a rotor 15, a stator 19, and a bit 16. Drilling fluid flows through fluid line 10 and mud motor 14 to bit 16. A portion of the drilling fluid is diverted via line 17 to tractor 11. When drilling fluid enters pressure chamber 12, piston head 13 drives bit 16 into the formation. The drilling fluid flowing through mud motor 14 induces rotation of power-section rotor 15 and connected bit 16. Thus, mud motor 14 uses the pressure differential across power-section rotor 15 to induce bit 16 to rotate whereas tractor 11 uses the pressure in chamber 12 to drive piston head 13 and bit 16 into the formation.
Because tractor 11 and mud motor 14 draw from a common hydraulic fluid line 10, an unstable operating condition in mud motor 14 may cause a corresponding instability in tractor 11, and vice versa. For example, during drilling operations, the BHA may encounter a formation having earth and rock that is particularly difficult to disintegrate. A bit 16 forced against this hard to drill formation tends to increase the torque required to turn the drill bit against the formation. The bit torque increase causes a resultant increase in the differential pressure across power section 18 of mud motor 14. As the pressure differential across mud motor 14 increases, the pressure of the drilling fluid in fluid line 10 upstream of mud motor 14 also increases. Tractor 11 receives this higher pressure drilling fluid from line 17 which is connected to fluid line 10. Because drilling fluid pressure and tractor thrust are directly related, this increased pressure causes tractor 11 to drive the bit 16 even harder against the formation and at a faster rate. This increase in tractor rate of advancement further contributes to the increase in the torque required to turn the bit 16, thereby creating a feed-back effect which may ultimately cause the bit to stall or shorten the operating life of BHA components such as mud motor 14.
Some systems incorporate shock absorbers or dampeners in BHAs just above the mud motors. These shock absorbers or dampeners are sometimes Belleville springs that reduce the spring rate of the BHA between the motor and the tools above. However, having the springs just above the mud motors increases the length of the drillstring and also requires extra connections. An additional spline for transmitting torque load is also required. Additionally, the tractor still pushes the bit by weight on bit and can have the same problems discussed above. The tractor, having dampeners on each anchor allows for each dampener to be reset whenever its anchor disengages the hole wall so that additional length of dampening movement can allow tractor rate of advancement to slow down to drilling rate. Also directional control ability of drill bit below is reduced due to lower bending rigidity, and also circumferential looseness of spline connections.
The present invention addresses these and related deficiencies in prior art systems discussed above.
The present invention features a thrust absorber interposed between a thrusting means and an anchoring means. Normally, the thrusting means and the anchoring means cooperate to axially displace a tube. In a preferred embodiment, the thrust absorber includes an enclosure that is fixed to the anchoring means and a retainer connecting to the thrusting means. Disposed within the enclosure is a biasing member that is configured to absorb thrust energy when a predetermined condition occurs. Particularly, the thrusting means can encounter an overthrust condition when the thrusting means imparts a thrust force to the tube, but the tube is not substantially axially displaced. When an overthrust condition occurs, the biasing member is compressed by the tube, and thereby absorbs the thrust that otherwise would have been imparted to the tube. Also, by absorbing the thrust, the pressure increase is substantially reduced. The reduction in pressure increase reduces the tractor advancement rate increase so that the tractor rate is modulated and makes the system more stable. Furthermore, for a bottom hole assembly having more than one thrusting means, a thrust absorber may be provided for each such thrusting means.
In a first and second alternative embodiment, the thrust absorbers additionally comprise two different configurations that restrict the speed of movement of the thrust absorbers. The thrust absorbers are especially restricted once the external load across the absorber is relaxed.
In a third alternative embodiment, the thrust absorber additionally comprises a second biasing member disposed within the enclosure. Particularly, the second biasing member restricts movement of the thrust absorber when the tube is displaced in a direction opposite that of the intended forward direction of the tractor. The second biasing member allows most of the length of the thruster stroke to be realized by preventing loss of stroke length due to movement of the thrust absorber.
The present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.